Specify behavior when sources are added to multiple Pollers or Poller-like constructs

[notgull]

As far as I can tell, from some testing that I just ran, when a source is registered into multiple Pollers, any events that originate from the source will appear on every Poller. Do we want to specify this behavior as guaranteed, or do we want to go in another direction.

The reason why I ask is because, as of Windows 8.1, there is a WSAEventSelect function that is stable, documented and would require no external dependencies over the current wepoll backend. However, it comes with the caveat that every socket can only be registered in one WSAEVENT at a time. If we decide that we want to guarantee the current general behavior, we could not switch to this.

Interaction with mio isn't an issue, since mio says that being registered to mio::Poll is exclusive.